Page 42 - Volume 3, Issue 2, July 2010

Pesticides in cereal-based baby foods

73

According to Document SANCO/3131/2007

of the European Commission, the potential

for matrix effects should be assessed dur-

ing the validation of the method (5). The ef-

fect can be due to different reasons e.g. the

presence of a blank, due to solvent and/or

reagents or the presence of a compound in

the sample that contributes to the analyti-

cal signal (7). Matrix-induced enhancement

is a phenomenon commonly found in gas

chromatographic analysis of pesticides in

food (5) and has been noticed in the anal-

ysis of these compounds by either GC-ECD

(11) or GC-NPD (5). For this purpose, matrix-

matched standard solutions (including ma-

trix blanks) were used. An aliquot of

2ml of

control sample extracts were evaporated to

dryness by a stream of N

2

. After evaporation

1 ml of a standard solution at the desired

concentration prepared in 2,2,4-trimethyl-

pentane/toluene (90/10) was added. The fi-

nal solution was reconstructed in an ultra-

sonic bath for 30 sec.

Preparation of fortified samples

Control samples were prepared from

organically produced cereal-based in-

fant food. Aliquots of 5 g portion of cere-

al-based infant food sample were fortified

at two levels. The lowest fortification lev-

el was 3 μg/kg and the second fortification

level was 10 times higher (30 μg/kg). For

validating the method according to SANCO

2007/3131, a minimum of five replicates is re-

quired. Therefore, five replicates were used

for each level.

Sample preparation

For the extraction of the pesticides, the

protocol of QuEChERS method concerning

commodities with high fat content was fol-

lowed (2, 3). According to this method, a 5 g

portion of the cereal-based infant food was

weighted in a 50 ml PTFE centrifuge tube,

and 10 ml of water were added. The water

should be at low temperature (< 4

o

C) to com-

pensate for the heat development caused

by the addition of the salts that follows. A

short vibration using a Vortex mixer (K-550-

GE, Scientific industries inc. Bohemia, USA)

helped to disperse solvent and pesticides

well through the sample. For the extraction

of the pesticides, 10 ml of acetonitrile were

added and the tube was vigorously shak-

en for 1min. A mixture of 1 g of NaCl, 4 g of

MgSO

4

, 1 g of trisodium citrate dehydrate

and 1 g of disodium hydrogencitrate sesqui-

hydrate were added and the tube was vigor-

ously shaken for 1 or more minutes to pre-

vent coagulation of MgSO

4

. The sample was

then centrifuged (4000 rpm) for 5 min. An al-

iquot of 8 ml of the supernatant (acetonitrile

phase) was then taken and transferred into a

15 ml centrifuge tube and stored for at least

2 h in the freezer. Freezing-out helps to part-

ly remove some additional co-extractives

with limited solubility in acetonitrile, while

the major part of fat and waxes solidify and

precipitate. An aliquot of 6 ml of the still

cold acetonitrile phase was transferred into

a 15 ml centrifuge tube containing 250 mg

of PSA and 750 mg of MgSO

4

and the tube

was shaken vigorously for 30 sec and cen-

trifuged for 5 min at 4000 rpm. An aliquot

of 5 ml of the cleaned-up extract was trans-

ferred into a screw cup storage vial, taking

care to avoid sorbent particles of being car-

ried over. The extract was slightly acidified

by adding 50 μl of a 5% formic acid solution

in acetonitrile. An aliquot of 2 ml of the ex-

tract was obtained, evaporated near dry-

ness and reconstituted in 1 ml of 2,2,4-tri-

methylpentane/toluene (90/10). The final

extract was sonicated in an ultrasonic bath

for 30 sec and transferred into autosampler

vials with a Teflon stopper.

Gas-chromatographic analysis

Determination was performed by the

use of an Agilent 6890

gas chromatograph,

with two splitless injectors,

a DB-5-MS col-

umn (30 m, 0.32 mm i.d. and 0.25 μm film

thickness) connected to the ECD and a DB-

17 MS column (30 m, 0.3 mm i.d. and 0.25

μm film thickness) connected to the NPD

and equipped with a Chemstation chro-

matography manager data acquisition and

processing software. For confirmation pur-

poses the two columns were swapped be-

tween the two detectors.